if the question is in form that why atomic mass of chlorine is fractional?

Dear user!

Your query is quite obvious as you have studied that atomic mass of any element depends upon the number of protons and neutrons present in it. Therefore, the atomic mass of a substance should always be a whole number.

But chlorine is an exception to this as it is having a fractional atomic mass i.e., 35.5.

This is because; in nature chlorine exists in two isotopic forms. These are . These two isotopes have a natural abundance ratio of 3:1. This means that 75.4% of chlorine found in nature exists as isotope, and the rest 24.6% exists as  isotope.

To calculate the average mass of the element, mass as well as the abundance of all isotopic forms are taken into consideration.

Hence, the mass of chlorine can be calculated as follows:

I hope, now it is clear that the value, 35.46 u, indicates the average mass of chlorine atom due to the presence of both the isotopes in a certain amount.

All the best!!